This is really annoying for some of us who use multiple languages. Swiftkey can predict words from as many languages as you enable. But (Google) spell check works on only one. Swiftkey gives you no option to disable it. So i could turn spellcheck off in Google keyboard but not in swiftkey!
Funnily, most solutions on the net require ADB or simply don’t work.
For MIUI7, the solution is super simple – disable Google Keyboard under settings / installed apps. End of discussion.
Source: https://www.reddit.com/r/galaxys4/comments/32v5tg/does_anyone_have_this_text_underlining_red/ (but the solution was like buried in the comments – hence this post!)